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| <h3 class="section">29.2 The Persona of a Process</h3> |
| |
| <p><a name="index-persona-3295"></a><a name="index-effective-user-ID-3296"></a><a name="index-effective-group-ID-3297"></a><a name="index-supplementary-group-IDs-3298"></a> |
| <!-- When Hurd is more widely used, explain multiple effective user IDs --> |
| <!-- here. -zw --> |
| At any time, each process has an <dfn>effective user ID</dfn>, a <dfn>effective |
| group ID</dfn>, and a set of <dfn>supplementary group IDs</dfn>. These IDs |
| determine the privileges of the process. They are collectively |
| called the <dfn>persona</dfn> of the process, because they determine “who it |
| is” for purposes of access control. |
| |
| <p>Your login shell starts out with a persona which consists of your user |
| ID, your default group ID, and your supplementary group IDs (if you are |
| in more than one group). In normal circumstances, all your other processes |
| inherit these values. |
| |
| <p><a name="index-real-user-ID-3299"></a><a name="index-real-group-ID-3300"></a>A process also has a <dfn>real user ID</dfn> which identifies the user who |
| created the process, and a <dfn>real group ID</dfn> which identifies that |
| user's default group. These values do not play a role in access |
| control, so we do not consider them part of the persona. But they are |
| also important. |
| |
| <p>Both the real and effective user ID can be changed during the lifetime |
| of a process. See <a href="Why-Change-Persona.html#Why-Change-Persona">Why Change Persona</a>. |
| |
| <p>For details on how a process's effective user ID and group IDs affect |
| its permission to access files, see <a href="Access-Permission.html#Access-Permission">Access Permission</a>. |
| |
| <p>The effective user ID of a process also controls permissions for sending |
| signals using the <code>kill</code> function. See <a href="Signaling-Another-Process.html#Signaling-Another-Process">Signaling Another Process</a>. |
| |
| <p>Finally, there are many operations which can only be performed by a |
| process whose effective user ID is zero. A process with this user ID is |
| a <dfn>privileged process</dfn>. Commonly the user name <code>root</code> is |
| associated with user ID 0, but there may be other user names with this |
| ID. |
| <!-- !!! should mention POSIX capabilities here. --> |
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